Process for improving water-whitening resistance of pressure sensitive adhesives

ABSTRACT

A process is disclosed for improving the water-whitening resistance of a pressure sensitive adhesive containing an aqueous latex emulsion and water-soluble ions by removing the water-soluble ions and adjusting the pH of the pressure sensitive adhesive formulation to at least about 6.0. The preferred method of removing the water-soluble ions is to contact either the aqueous latex emulsion, the formulated pressure sensitive adhesive containing the aqueous latex emulsion, or both with ion exchange resin. The product produced by the process is also disclosed.

This is a divisional of application Ser. No. 888,563, filed May 22,1992, now U.S. Pat. No. 5,286,843.

FIELD OF THE INVENTION

This invention is directed to a process for improving aqueous latexemulsion coatings, and more particularly to a process for improving thewater-whitening resistance of pressure sensitive adhesives.

BACKGROUND OF THE INVENTION

Pressure sensitive adhesives may be used with transparent backings, suchas for example polyvinyl chloride, polyethylene, polypropylene,polyester and the like, to form transparent labels and decals. Largelabels and decals are applied to the sides of trucks, store windows andthe like. These surfaces are often squeegied clean prior to theapplication of the label or decal. This leaves a thin layer of water onthe surface. The wet surface allows for the adjustment or positioning ofthe label or decal. In cases where the backings are transparent, thiscan result in the development of water-whitening or hazing in thepressure sensitive adhesive, which persists until the system issubstantially dry. This drying period may be unacceptably long. Forexample, a large label, such as that applied to the side of a truck, maytake several hours to dry after application of the label. Because thelabels and decals are used in advertising and displays, thiswater-whitening, although only temporary and not detrimental to thepressure sensitive adhesive's ultimate performance, is not aestheticallyacceptable.

Although not intending to be bound by theory, I believe that thewater-whitening or hazing is caused by the diffusion of water acrosslatex particle boundaries into the interstitial areas in the driedcoating forming pockets of water. This gives rise to an osmotic pressurewhich acts to push the latex particles apart. When the pockets of waterbecome large enough, they scatter visible causing the water-whitening orhazing.

Normally when the aqueous latex emulsion dries, the latex particlescoalesce to form a film and the other components of the adhesiveformulation which may be ionically-charged and water-soluble becometrapped in the interstitial areas of the film. These materials, such asfor example, dispersants, surfactants, coalescents, wetting agents,rheology modifiers, thickeners, drying retarders, antifoaming agents,preservatives, heat stabilizers, ultraviolet light absorbers and thelike, may attract water because they may be more hydrophilic than theemulsion particles and become the driving force for the migration of thewater into the interstitial areas causing the water-whitening problem.Solvent-based adhesives do not experience this water-whitening problem,however they present safety, health and environmental concerns. Bycontacting the aqueous latex emulsion used in the adhesive formulationor the formulated adhesive with an ion exchange resin, I have found thatwater-soluble ions may be removed so that the water-whitening resistanceor hazing of the pressure sensitive adhesive is improved.

DESCRIPTION OF THE PRIOR ART

U.S. Pat. No. 3,033,811 concerns pigmented thermosetting coatingcompositions which can be applied from aqueous media to the surfaces tobe coated. It teaches the removal of water-soluble ionizable compoundspresent in the dispersion by dialysis, deionization with ion exchangeresins or precipitation to increase the water-resistance of the finalbaked coating.

U.S. Pat. No. 3,494,878 teaches an ion exchange resin of anionic orcationic types or mixtures thereof which are incorporated in aqueouscoating compositions the binder of which comprises a dispersed emulsionpolymer. The dear or pigmented coating compositions resist staining whenthe coatings are applied over cedar, redwood, mahogany and rustysurfaces.

U.S. Pat. No. 3,509,084 teaches a basic aqueous dispersion containingmetallic ions which is contacted with a strongly acidic ion exchangeresin charged with ammonium ions to produce an aqueous dispersionsubstantially free of metallic ions.

U.S. Pat. No. 4,775,588 teaches a hydrophilic film-forming compositionwhich comprises a resin paint comprising a resin binder and a paintvehicle with a solid ion exchange resin powder dispersed in the vehicle.The incorporation of the ion exchange resin powder into an undercoattreatment provides improved corrosion resistance.

U.S. Pat. No. 4,853,422 teaches a paint coating comprising a self-curingemulsion polymer having functional carboxyl groups or hydroxyl group aswell as alkylol acrylamide where said self-curing emulsion polymer ision-exchanged to remove cations therefrom and the pH is less than 2.5.

U.S. Pat. No. 4,900,592 teaches a paint coating containing athermosetting polymeric binder based on an ion-exchange treated,thermosetting functional latex polymer. The functional latex polymercomprises emulsion polymerized ethylenic monomers including a β-hydroxyester monomer, but specifically excluding amine monomers, to produce afunctional latex. The functional latex is then treated in anion-exchange process to produce a low pH functional latex adapted toself-crosslink by transesterification with respective β-hydroxy estergroups in the latex.

It is an object of the invention to provide a process for improving thewater-whitening resistance of pressure sensitive adhesives by removingthe water-soluble ions from the latex polymer emulsion or pressuresensitive adhesive formulation containing latex polymer emulsion withoutadversely affecting the pressure sensitive adhesives performanceproperties.

It is a further object of the present invention to provide a pressuresensitive adhesive with improved water-whitening resistance.

Other objects and advantages will become apparent from the followingdescription and claims.

SUMMARY OF THE INVENTION

This invention is a process for improving the water-whitening resistanceof a pressure sensitive adhesive containing an aqueous latex emulsion byremoving the water-soluble ions either from the aqueous latex emulsionor the formulated pressure sensitive adhesive containing the aqueouslatex emulsion and adjusting the pH of the pressure sensitive adhesiveformulation to at least about 6.0. It is preferred that thewater-soluble ions are removed by contacting the aqueous latex emulsion,the formulated pressure sensitive adhesive or both with ion exchangeresins. The product of this process is a pressure sensitive adhesivewith improved water-whitening resistance. The process is particularlyuseful for producing a pressure sensitive adhesive which is used toadhere clear labels and decals to surfaces, such as store windows andthe sides of vehicles. The process solves the water-whitening problemtypically encountered by the use of water on the surface to aid in thepositioning of the clear label or decal. The process improves theaesthetic characteristics of the pressure sensitive adhesive withouthaving a detrimental effect on the functional characteristics.

DETAILED DESCRIPTION OF THE INVENTION

The term "adhesive" as used herein refers to a coating containing anaqueous latex polymer which when applied to surfaces joins them togetherand resists separation.

The term "pressure sensitive" as used herein refers to adhesives whichtypically do not undergo hardening after they have been applied tosurfaces and the joint formed.

The term "latex" as used herein refers to a water-insoluble polymerwhich may be prepared by conventional polymerization techniques such as,for example, by emulsion polymerization.

The aqueous latex polymers used in the pressure sensitive adhesives ofthis invention are prepared by emulsion polymerization techniques wellknown in the art. The aqueous latex polymer may be formed from anymonomer or mixture of monomers which yields a water-insoluble latex,film-forming polymer. The aqueous latex polymers used in the pressuresensitive adhesives of this invention typically have a glass transitiontemperature as measured via differential scanning calorimetry of lessthan about -10° C. The weight-average molecular weight of the aqueouslatex polymer may be adjusted through the addition of n-dodecylmercaptan during emulsion polymerization to give a suitable balance ofadhesive and cohesive strength.

A wide variety of monomers or mixture of monomers may be used to makethe aqueous latex polymer used in the adhesives of this invention. Forexample, acrylic ester monomers, including methyl acrylate, ethylacrylate, propyl acrylate, isopropyl acrylate, butyl acrylate, isobutylacrylate, secondary butyl acrylate, t-butyl acrylate, pentyl acrylate,neopentyl acrylate, hexyl acrylate, heptyl acrylate, octyl acrylate,isooctyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, isodecylacrylate, lauryl acrylate, bornyl acrylate, isobornyl acrylate, myristylacrylate, pentadecyl acrylate, stearyl acrylate and the like;methacrylic acid ester monomers, including methyl methacrylate, ethylmethacrylate, propyl methacrylate, isopropyl methacrylate, butylmethacrylate, isobutyl methacrylate, hexyl methacrylate, octylmethacrylate, isooctyl methacrylate, decyl methacrylate, isodecylmethacrylate, lauryl methacrylate, bornyl methacrylate, isobornylmethacrylate, myristyl methacrylate, pentadecyl methacrylate, stearylmethacrylate, phosphoethyl methacrylate and the like; acrylic acid,methacrylic acid, itaconic acid, maleic acid, fumaric acid, styrene,substituted styrenes, butadiene, acrylonitrile, ethylene, vinyl acetate,vinyl chloride, sodium styrene sulfonates, sodium vinyl sulfonate,acrylamide, methacrylamide and the like may be used.

The removal of water-soluble ions from the aqueous latex polymeremulsion or pressure sensitive adhesive formulation is critical to theoperation of this invention. The water-soluble ions may be removed by anumber of techniques, including, but not limited to, centrifugation,dialysis, precipitation and deionization with ion exchange resins. Thepreferred method of removing the water-soluble ions is to contact theaqueous latex polymer emulsion or the formulated pressure sensitiveadhesive containing the aqueous latex polymer emulsion with ion exchangeresin. Contacting the final pressure sensitive adhesive formulationcontaining the aqueous latex polymer with the ion exchange resin ispreferred.

The aqueous latex polymer emulsion or the pressure sensitive adhesiveformulation containing the aqueous latex polymer may be treated with ionexchange resin either

(1) by adding the ion exchange resin to the aqueous latex polymeremulsion or the pressure sensitive adhesive formulation containing theaqueous latex polymer; or

(2) by passing the aqueous latex polymer emulsion or the pressuresensitive adhesive formulation containing the aqueous latex polymer overa column bed of ion exchange resin.

When adding the ion exchange resin to the aqueous latex polymer emulsionor the pressure sensitive adhesive formulation containing the aqueouslatex polymer, the ion exchange resin must be mixed thoroughly to permitcontact of the ion exchange resin surfaces with the water-soluble ionsin the emulsion or adhesive. The contact can be effected by adding tothe aqueous latex polymer emulsion or the pressure sensitive adhesiveformulation containing the aqueous latex polymer from about 1% by weightto about 100%, preferably greater than about 2%, by weight of the ionexchange resin to the weight of the emulsion or adhesive. The admixturemay be agitated to facilitate contact between the ion exchange resin andthe water-soluble ions. The ion exchange resin may be removed from themixture by straining through a filter medium such as linen cloth, nylonmesh filter and the like to remove all of the ion exchange resin. Theprocedure may be repeated several times, if necessary, to remove thewater-insoluble ions from the aqueous latex polymer emulsion or thepressure sensitive adhesive formulation containing the aqueous latexpolymer.

The rate of from about 0.5 bed volume/hour to about 10 bed volumes/hour,of passing the aqueous latex polymer emulsion or the pressure sensitiveadhesive formulation containing the aqueous latex polymer over a columnbed of ion exchange resin is effective to remove the water-soluble ions.A rate of from about 1 bed volume/hour to about 6 bed volumes/hour ispreferred, with a rate of from about 1 bed volume/hour to about 3 bedvolumes/hour most preferred.

The preferred method of using the ion exchange resins comprises making amixture of a sulfonic add type cation exchanger in the acid form (toremove anions) and a quaternary anion exchange resin in the free baseform (to remove cations), and contacting the aqueous latex polymeremulsion or the pressure sensitive adhesive containing the aqueous latexpolymer to be deionized with the mixture. Such mixtures of cation andanion exchange resins are well-known to those skilled in the art as"mixed beds." Cation and anion exchange resins may also be used insuccession. Cation or anion exchange resins may also be used separately,however complete deionization may not be achieved.

Suitable anion exchange resins useful alone, in succession, or in mixedbeds include copolymers of styrene and divinyl benzene containing amineor quaternized amine functional groups and the like. Preferred are thoseresins in spherical bead form, including gel and macroporous beads.Examples of anion exchange resins are disclosed in U.S. Pat. Nos.2,591,573; 2,591,574; 2,597,439; 2,597,440; 2,630,427; 2,630,428,2,630,429; and 2,402,384 which are hereby incorporated herein byreference.

Suitable cation exchange resins useful alone, in succession, or in mixedbeds include copolymers of styrene and divinyl benzene containing havingstrong acid functional groups such as sulfonic acid groups. Preferredare those resins in spherical bead form, including gel and macroporousbeads. Examples of cation exchange resins are disclosed in U.S. Pat.Nos. 4,224,415; 4,256,840; and 4,382,124 which are hereby incorporatedherein by reference.

More preferred for mixed bed use are anion exchange resins and cationexchange resins having different hydraulic densities, such that the twotypes of resins in the mixed bed will separate during reversed liquidflow through the bed, to allow easy regeneration of the two resins.

The pH of the pressure sensitive adhesive formulation containing theaqueous latex polymer emulsion after it has been deionized affects theoperation of this invention. The pressure sensitive adhesive formulationcontaining the aqueous latex polymer must have a pH of greater thanabout 6, with higher pH's preferred. In practice, the pH is limited onlyto an upper level which does not thicken the pressure sensitive adhesiveformulation containing the aqueous latex polymer, above a viscosityuseful in conventional coating techniques. Films cast from deionizedpressure sensitive adhesive formulations having pH's less than about 6exhibit poor water-whitening resistance. To adjust the pH of thepressure sensitive adhesive formulation, non-permanent bases, such asfor example ammonium hydroxide, and the like may be added. Permanentbases, such as for example sodium hydroxide, typically are not suitableto adjust the pH because they add permanent ions which are detrimentalon water whitening resistance.

The aqueous latex polymer emulsion may have a level of solids from about40% to about 70%.

The pressure sensitive adhesive may be applied to substrates viaconventional coating techniques including gravure, roller and slot-diecoaters and the like.

The following examples are intended to illustrate the invention; theyare not intended to limit the invention as other applications of theinvention will be obvious to those of ordinary skill in the art.

    ______________________________________                                        ABBREVIATIONS:                                                                ______________________________________                                        g                     grams                                                   BA                    butyl acrylate                                          EA                    ethyl acrylate                                          AA                    acrylic acid                                            ______________________________________                                    

EXAMPLE 1 Preparation of Pressure Sensitive Adhesive Formulations

To a 1 pint can equipped with a laboratory stirrer, 100 g latex polymeremulsion was added:

    ______________________________________                                        Pressure Sensitive Adhesive                                                   Formulation      Composition of Latex Polymer                                 ______________________________________                                        1                49 BA/49 EA/2 AA                                             Comparative 1    49 BA/49 EA/2 AA                                             2                59 BA/39 EA/2 AA                                             Comparative 2    59 BA/39 EA/2 AA                                             3                75 BA/22 EA/3 AA                                             Comparative 3    75 BA/22 EA/3 AA                                             ______________________________________                                    

At moderate agitation, 0.012 g Triton® X-45 surfactant, 0.3 g Emcol®4500 surfactant and 0.6 g Drew® T-4201 defoamer were added and mixed.The pressure sensitive adhesive formulation was mixed for an additional15 minutes. The pH of the final pressure sensitive adhesive formulationwas adjusted to a range of 6.5 to 7.0 using 28% by weight solution ofammonium hydroxide.

EXAMPLE 3 Deionization of Pressure Sensitive Adhesive Formulations

Amberlite® IRN-150 ion exchange resin, a highly purified mixture ofstrongly acidic cation resin and strongly basic anion exchange resinwith the mixture containing an equivalent of hydrogen ion for eachequivalent of hydroxide ion, was added gradually to the pressuresensitive adhesive formulation and stirred to avoid the formation oflarge dumps. The mixture was stirred for about 2 hours and then filteredthrough a 325 mesh screen. The procedure was repeated, where necessary.The pressure sensitive adhesive formulations which were not deionizedare designated as comparatives.

EXAMPLE 4 Preparation of Test Specimens

The pressure sensitive adhesive formulations were direct-coated ontoMylar® film using a Bird applicator and dried at 180° F. for 15 minutesto give a 1 mil dry film thickness. The test specimens were then nippedto untreated polypropylene. The test specimens were cut into thefollowing sizes:

    ______________________________________                                        Test                Dimensions                                                ______________________________________                                        180° peel adhesion                                                                         1 inch wide strips                                        Polyken tack        1 inch wide strips                                        Shear resistance testing                                                                          1 inch wide strips                                        Water-whitening resistance                                                                        1 inch by 5 inch strips                                   ______________________________________                                    

The test specimens were conditioned at 77° F. and 55 % relative humidityovernight prior to all testing.

EXAMPLE 5 Water-whitening Resistance Testing

To evaluate water-whitening resistance, the test specimens preparedaccording to Example 4 were placed a 4 ounce glass jar filled with tapwater equilibrated to room temperature. The degree of opacity wasvisually rated at 5 minute intervals for a total of 90 minutes, usingthe following rating system:

    ______________________________________                                        Rating          Description                                                   ______________________________________                                        5               completely transparent                                        4               slight whitening                                              3               moderate whitening                                            2               severe whitening                                              1               opaque                                                        ______________________________________                                    

The results are shown in Tables 5.1.

                  TABLE 5.1                                                       ______________________________________                                        Water-whitening Resistance                                                                   Pressure Sensitive                                                            Adhesive Formulation                                           Time (minutes)   Comparative 2*                                                                            2*                                               ______________________________________                                        1                5           5                                                6                5           5                                                11               4           5                                                16               4           5                                                21               4           5                                                26               3           4                                                31               3           4                                                130              2           3                                                240              1           2                                                ______________________________________                                         *Note:                                                                        Formulated with 1.5 parts per hundred parts latex polymer of a mixture of     10 parts by weight of 59BA/39EA/2AA; 0.1 part by weight Triton ® X45      surfactant; 0.3 part by weight Emcol ® 4500 surfactant; and 5 parts b     weight Drew ® T4201defoamer                                          

EXAMPLE 6 Effect of Deionization Sequence

The effect of deionization before and after formulation of the pressuresensitive adhesive was evaluated. The results are shown in Table 6.1.Deionization after formulation shows better water-whitening resistancethan deionization before formulation.

                  TABLE 6.1                                                       ______________________________________                                        Water-whitening Resistance                                                            Pressure Sensitive Adhesive Formulation 2*                            Time      Deionization before                                                                         Deionization after                                    (minutes) formulation   formulation                                           ______________________________________                                        1         5             5                                                     6         5             5                                                     11        5             5                                                     16        5             5                                                     21        4             5                                                     26        4             5                                                     31        4             5                                                     130       3             4                                                     240       2             2                                                     ______________________________________                                         *Note:                                                                        Formulated with 1.5 parts per hundred parts latex polymer of a mixture of     10 parts by weight of 59BA/39EA/2AA; 0.1 part by weight Triton ® X45      surfactant; 0.3 part by weight Emcol ® 4500 surfactant; and 5 parts b     weight Drew ® T4201defoamer                                          

EXAMPLE 7 Effect of Ion Exchange Resin Weight and Repeated Treatment

The effect of ion exchange resin weight and repeated treatment wereevaluated. The results are shown in Table 7.1. The results show thatcontacting from about 1% by weight to about 100%, preferably greaterthan about 2%, by weight of the ion exchange resin based on the weightof the pressure sensitive adhesive formulation is effective to improvethe water-whitening resistance. When the procedure was repeated at the2% by weight level an improvement in the water whitening resistance wasalso seen.

                  TABLE 7.1                                                       ______________________________________                                        Water-whitening Resistance                                                    Pressure Sensitive Adhesive Formulation 2*                                            Weight % Ion Exchange Resin                                                   30  20        10    2      2   0                                      Time      How many times?                                                     (minutes) 2     2         2   2      1   0                                    ______________________________________                                        1         5     5         5   5      5   5                                    6         5     5         5   5      5   5                                    11        5     5         5   5      5   5                                    16        5     5         5   5      5   5                                    21        5     5         5   5      5   4                                    26        5     5         5   5      5   4                                    31        5     5         5   5      4   3                                    36        5     5         5   5      4   3                                    90        5     5         5   4      4   3                                    210       4     5         4   3      3   2                                    360       3     4         2   2      2   1                                    ______________________________________                                         *Note:                                                                        Formulated with 1.5 parts per hundred parts latex polymer of a mixture of     10 parts by weight of 59BA/39EA/2AA; 0.1 part by weight Triton ® X45      surfactant; 0.3 part by weight Emcol ® 4500 surfactant; and 5 parts b     weight Drew ® T4201defoamer                                          

EXAMPLE 8 Effect of pH on Water-Whitening Resistance

The effect of pH of the final formulated pressure sensitive adhesive onthe water-whitening resistance was evaluated. The results are shown inTable 8.1. The pressure sensitive adhesive formulation which had beendeionized but which had not been readjusted to a pH greater than about6.0 did not demonstrate an improved resistance to water-whitening.

                  TABLE 8.1                                                       ______________________________________                                        Water-whitening Resistance                                                           Pressure Sensitive Adhesive Formulation                                         Comparative 3  3         3                                           Time     not deionized  deionized deionized                                   (minutes)                                                                              pH = 6.7       pH = 1.75 pH = 6.5                                    ______________________________________                                         1       5              4         5                                            3       5              3         5                                            5       4              3         5                                           10       4              3         5                                           15       4              2         4                                           20       4              2         4                                           30       3              2         4                                           35       3              2         4                                           95       3              1         4                                           ______________________________________                                    

EXAMPLE 9 Pressure Sensitive Adhesive Property Performance Testing

The following tests were performed to demonstrate that deionization doesnot adversely affect the pressure sensitive adhesive performanceproperties of formulations. The results for the pressure sensitiveadhesive property testing are shown in Table 9.1.

180° Peel Adhesion Test

The 180° peel adhesion test was performed according to PressureSensitive Tape Council Method Number 1 using dwell times of 20 minutesand 72 hours.

Polyken Tack Test

The Polyken tack test was performed using a Polyken Tack Tester fromTesting Machines, Inc. with a 1 second dwell time, 1 centimeter/secondprobe withdrawal speed and a 20 gram weight.

Shear Resistance Test

The shear resistance test was performed according to Pressure SensitiveTape Council Method Number 7, using a 1 inch by 1 inch overlap area and1 kilogram weight.

                  TABLE 9.1                                                       ______________________________________                                        180° Peel Adhesion (ounces/inch)                                                  20 minute dwell                                                                              72 hour dwell                                       ______________________________________                                        1          54             73                                                             adhesive failure mode                                                                        adhesive failure mode                               Comparative 1                                                                            58             71                                                             adhesive failure mode                                                                        adhesive failure mode                               ______________________________________                                                   Polyken Tack (grams)                                               ______________________________________                                        1          715                                                                Comparative 1                                                                            655                                                                ______________________________________                                                   Shear Resistance                                                   ______________________________________                                        1          86 light cohesive failure mode                                     Comparative 1                                                                            99 cohesive failure mode                                           ______________________________________                                    

The results showed that deionization to improve water-whitening did notadversely affect the pressure sensitive adhesive formulationsperformance properties. In the 180° Peel Adhesion Test, the deionizedformulation showed essentially the same peel strength values and thesame type of failure mode as the comparative formulation which had notbeen deionized. In the Polyken Tack Test, the deionized formulationshowed only a slight increase in the level of tack as compared to thecomparative formulation which had not been deionized. In the ShearResistance Test, the deionized formulation showed essentially the sameshear strength value and type of failure mode as the comparativeformulation which had not been deionized.

What is claimed is:
 1. A product produced by the process comprising thesteps of:(a) using an ion exchange resin to remove water-soluble cationsand the water-soluble anions from a pressure sensitive adhesiveformulation containing an aqueous latex emulsion; and (b) adjusting thepH of the pressure sensitive adhesive formulation to at least about 6.0.2. The product of claim 1 wherein the water-soluble ions are removed bya sulfonic acid type cation exchange resin in the acid form and aquaternary anion exchange resin in the free base form.
 3. The product ofclaim 1, wherein the pH is adjusted using a base that does not addpermanent ions to the pressure sensitive adhesive formulation.
 4. Theproduct of claim 1, wherein the pH is adjusted using ammonium hydroxide.